1. Field of the Invention
The invention relates to a road friction coefficient estimating unit for a vehicle and a road friction coefficient estimating method for a vehicle.
2. Discussion of Background
Various road friction coefficient estimating methods are described in, for example, Japanese Patent Application Publication No. 2006-56388 (JP 2006-56388 A).
A steering assist electric motor used in a vehicle electric power steering system is provided, for example, in a steering gear device formed of a rack and a pinion or in a column mechanism arranged between a steering wheel and a steering gear device. In each case, an operation of a steered shaft coupled to steered wheels is assisted by the electric motor.
As the amount by which a tire is steered with respect to the travelling direction of a vehicle is increased by turning the steering wheel by a larger amount, a reaction force from the road surface increases and a larger force is required to turn the steering wheel. Therefore, the above-described electric motor is configured such that, as a steering torque of the steering wheel increases, a steering assist torque for assisting a steering operation is increased by increasing a motor current.
Among forces that the vehicle receives from a ground surface via each tire, a force with which the ground surface pulls the tire in the lateral direction (direction perpendicular to the travelling direction of the vehicle) is called “cornering force”, and a force with which the ground surface pulls the tire in a direction perpendicular to the direction of the tire is called “tire lateral force”. In addition, a torque around a ground contact point of the tire is called “self-aligning torque”. That is, the tire that has a tire sideslip angle (angle between the travelling direction of the vehicle and the direction of the tire) and that is generating a cornering force (or lateral force) generates a moment (self-aligning torque) for returning the tire sideslip angle to zero degrees. The self-aligning torque is expressed by the product of a cornering force by a trail length (distance from a ground contact center to a point of application of the cornering force). Here, the trail length is expressed by the sum of a pneumatic trail and a caster trail.
In addition, the ratio of a cornering force to a tire sideslip angle is called “cornering power”. When a tire sideslip angle is zero or falls within a small range close to zero, a cornering force and a tire sideslip angle are in a linear proportional relationship, and the “cornering power” takes a constant value. However, as the tire sideslip angle becomes large, the rate of increase in cornering force gradually decreases. A range in which the rate of increase in concerning force gradually decreases is called a saturated range. In the saturated range, the ratio of the cornering force to the tire sideslip angle is a function of a road friction coefficient.
A steering mechanism of the power steering system includes a pinion shaft coupled to a steering column, and a rack shaft that is meshed with the pinion shaft and that serves as a steered shaft extending in the lateral direction of the vehicle. Knuckle arms are respectively coupled to a pair of end portions of the rack shaft via tie rods, and the direction of steered wheels including tires is changed by the knuckle arms.
A value obtained by multiplying a rack axial force in the lateral direction, which acts on the rack shaft, by a knuckle arm length is equal to the self-aligning torque of a steered tire. The self-aligning torque is expressed by the product of a cornering force by a trail length, as described above. Thus, the rack axial force is obtained by multiplying a cornering force by the ratio of the trail length to the knuckle arm length.
When the rack axial force is obtained through calculation with the use of a vehicle model, if the cornering force of a tire is in a saturated range, a term that includes a road friction coefficient appears in a rack axial force estimating equation (or a cornering force estimating equation). It is possible to measure the other terms used in the rack axial force estimating equation by using various sensors mounted in the vehicle. However, it is difficult to measure a road friction coefficient.
Therefore, it is possible to apply a vehicle model only in a linear range in which a road friction coefficient is not used. However, it is difficult to apply a vehicle model in a saturated range in which a road friction coefficient is used.